1. Field of the Invention
The present invention relates to a forming apparatus and a method for forming a web from stock having an ash content above 20%. More particularly, the present invention relates to a forming apparatus using a high ash content stock in which the resultant web is super calendered in order to provide a printable surface thereon.
2. Information Disclosure Statement
In the papermaking art it is known to supply a high ash content stock from a headbox onto a generally horizontally disposed forming wire. The arrangement is such that water initially drains downward from the stock through the wire. A top wire cooperates with the forming wire downstream relative to the initial formation section and a suction box within the top wire draws water upwardly away from the stock so that a web having generally uniform surface characteristics on both sides thereof is formed.
Nevertheless, as machine speeds have increased, the aforementioned initial forming section and subsequent top wire removal has resulted in a certain amount of two sidedness in the resultant web. Two sidedness in the present specification means that the surface characteristics of one surface of the resultant web is substantially different from the surface characteristics of the opposite surface of the web. Such two sidedness causes problems when both sides of the resultant web are to be printed thereon.
Machine speeds in the region of 1,600 meters per minute which is equivalent to 5,200 feet per minute are envisaged and at such speed, even removal or balanced removal of water within the stock from both surfaces thereof is essential.
Accordingly, the present invention provides a vertical former which includes a pair of wires which cooperate together to define therebetween a vertical forming section. A headbox is disposed closely adjacent to the lower or upstream end of the forming section so that the stock is ejected from the headbox upwardly into the forming section.
A curved shoe means is disposed immediately downstream relative to the upstream end of the forming section. Such curved shoe means has a very large radius of curvature within the range of 250 to 350 inches and preferably 300 inches. The arrangement is such that water is removed centrifugally away from the curved shoe means through a first wire, while a further portion of water is removed through the curved shoe means through the second wire.
A dewatering means is disposed downstream relative to the curved shoe means with the dewatering means having a radius of curvature which is opposite to the radius of the curvature of the curved shoe means. The dewatering means is structured such that the amount of water removed through the first wire by the dewatering means and by the curved shoe means is approximately equal to the volume of water removed through the second wire through the curved shoe means and through the second wire as the stock and wires move past the dewatering means.
Accordingly, the amount of water removed from the stock is evenly balanced as the stock is formed into a web so that the ash content or clay of filler within the stock is evenly distributed throughout the formed web. Subsequently, when the formed web is super calendered, the resultant calendered web exhibits surface characteristics similar to those of a coated web.
More particularly the resultant web is known in the art as super calendered Roto paper and super calendered Offset paper. Such papers contain mechanical pulp and clay fillers. The level of the fillers is higher than with normal paper production and may be in the region of 20 to 50 percent ash.
The Aforementioned web when super calendered has a high gloss together with a smooth printing surface which gives a very superior product which in many ways is comparable to a coated paper.
Therefore, the present invention provides a paper which requires no coating.
The prior production of the aforementioned supercalendered high ash content papers has been limited by forming speed, level of clay fillers and the dissimilarity of the surfaces or the resultant web.
The super calendering former of the present invention over comes the limitations of speed and can form sheets with a 31% filler level whereas current machines have a maximum of between 26 and 28% filler level. The present invention provides a sheet in which both sides thereof have very similar and even surface characteristics. The ash content at the respective surfaces thereof being approximately the same.
The present invention provides a curved shoe means which may have three blades approximately six inches wide in a machine direction with a spacing of approximately 2 inches between each blade. Such spacing of the three blades or shoes combined with the very large radius of curvature of the curved shoe means gives low intensity forming pulses to the stock so that the stock is gently dewatered during the initial or upstream portion of the forming section.
Therefore, it is a primary objective of the present invention to provide a forming apparatus which overcomes the aforementioned disadvantages of the prior art proposals and which makes a considerable contribution to the art of forming a web from stock.
Another object of the present invention is the provision of a web from a high ash content stock such that during subsequent supercalendering of the resultant web, even or similar surface characteristics of the web on both sides thereof are attained.
Other objects and advantages of the present invention will be readily apparent to those skilled in art by a consideration of the detailed description contained herein after taken in conjunction with the annexed drawings.